This invention pertains to a method and apparatus for minimizing visual perception of the raster lines on a photographic film recording of a video or TV monitor picture.
As is well known, when a viewer is near a video monitor, the horizontal scan lines can be seen on the face of the picture tube. If the viewer stays far enough away from the tube, however, the horizontal lines can no longer be resolved by the eyes and the picture appears rather uniform instead of appearing to be composed of a multiplicity of horizontal lines.
The present invention is especially concerned with obscuring the horizontal raster lines in a photograph of a picture taken of the screen of a video monitor tube when a static video picture is being displayed. An example of photographically recording static pictures on a video monitor tube arises in the medical field where video monitors are used to display converted x-ray images. In x-ray diagnostic systems, an x-ray image is converted to an optical image with an image intensifier and the optical image is viewed with a video camera. The resulting video signals may be stored in the video recorder and the signals from the recorder may be used to drive a video monitor. A camera is directed toward the picture tube face plate to enable photographic recording of any static image that is derived from the recorder and displayed on the monitor. Of course, when a conventional video system is used, the scan raster lines are also recorded on the film with such good definition as to be very distracting to anyone studying the photograph for detailed information.
One method that has been proposed for reducing visualization of the raster lines is to oscillate the scanning beam over the vertical distance of one pair of scan lines as the scanning beam progresses horizontally across the picture tube. This requires applying a high frequency signal to the scanning coil or electrodes of the video display tube such that the space ordinarily occurring between the scan lines appears to be filled and, accordingly, perception of the lines is reduced. However, this prior method has substantial technical and economic problems associated with it. It is not easy to control beam dithering with such precision as to avoid loss of picture information. This approach also increases the losses in the magnetic deflection coil and it wastes electric power.